Droplet-based technology is a tool with promising applications in the fields of biology, biotechnology, computation, and chemical analysis. In this technology, droplets are formed within an inert and immiscible carrier fluid or continuous phase at rates of several kHz using microfluidic techniques and devices. Once formed, these droplets can then be combined, split, and selected in any number of downstream steps. The speed, small volumes, and discrete partitioning that are characteristic of the technology have proven useful in many areas, such as those involving analyzing genetic material, screening large libraries of chemical compounds, and evolving cells and enzymes.
One application of this technique involves the encapsulation of individual cells within picoliter-scale monodisperse droplets. This enables the quantitative experimentation of large cell populations on a single-cell basis. Another application can be found in Droplet Digital PCR, in which the polymerase chain reaction samples are diluted and partitioned into a plurality of different reactions, such that each reaction contains at most one copy of the target nucleotide sequence to be amplified. As a result, a determination can be made of the original copy number of a DNA molecule by counting the number of reactions in which a successful PCR amplification occurs. Further applications and devices for droplet-based technologies are discussed in Koster S, Angile F E, Duan Agresti J J, Wintner A, Schmitz C, Rowat A C, Merten C A, Pisignano D, Griffiths A D, and Weitz D A. Drop-based microfluidic devices for encapsulation of single cells. Lab on a Chip 2008; 8: 1110, which is entirely incorporated herein by reference for all purposes.
One of the primary challenges with these techniques and applications is the variability in distribution of material among the discrete droplets. Because the distribution is essentially random, droplets can be formed that contain a higher or smaller amount of material than desired. For example, in applications requiring one particle per droplet, significant percentages of the droplet population generated may instead contain multiple particles or no particles at all.
A proposed solution, described in Edd J F, Di Carlo D, Humphry K J, Koster S, Irimia D, Weitz D A, and Toner M. Controlled encapsulation of single-cells into monodisperse picoliter droplets. Lab on a Chip 2008; 74: 61402, which is entirely incorporated herein by reference for all purposes, is to regulate the flow of cells entering a droplet generating device. This regulation is such that the cells are evenly spaced with one another, and that they enter the droplet generator at a frequency that precisely matches that of droplet formation.
An alternate solution, described in Abate A R, Chen C-H, Agresti, J J, and Weitz D A. Beating Poisson encapsulation statistics using close-packed ordering. Lab on a Chip 2009; 9: 2628-2631, which is entirely incorporated herein by reference for all purposes, is to use deformable particles that are closely packed. These particles comprise a compliant gel that has enough flexibility in its structure to prevent clogging of channels. In this way, the volume fraction of the particles can be increased beyond what would otherwise be practical, and the resulting efficiency of particle encapsulation can be increased.